add basics of Cut mouse mode, no functionality yet. Icon images are still required
add functionality for cut tool
Add Canvas::re_enter() which picks the current item again based on mouse pointer position and generates an enter event for it
tentative use of "c" as test binding for scissors/cut mouse mode (will possibly revert)
Use Canvas::re_enter() after changing mouse mode so we get the right cursor no matter what item we're over; fix return value for button press in cut mode; never change selection in cut mode
fix potential crash if certain Canvas methods are used before its window is realized
add new variant of EditorCursor that creates an insensitive full-canvas-height line, and fix destructor leak
use an EditorCursor when dragging in cut mode to indicate cut position